In the operation of automatic transmissions known from the operation of automobiles, switching elements implemented as clutches or brakes are operated depending on a driver-sided, currently selected operating gear, as for example “forward-drive,' “reverse-drive,” neutral,” or “park” and a current working condition of the vehicle via an hydraulic control. In order to reduce the required design envelope, such automatic transmissions are activated by a so-called electrified hydraulic switching device, by way of which a driver-desired input requirement is transferred via an electrically active connection between an operator control provided for a driver in the interior of the automobile to an electronic-hydraulic controller in the area of the transmission of the automobile.
An electronic-hydraulic controller of an automatic switching automobile transmission is known from DE 198 58 540 A1, with which the active connection between operator controls on the part of the driver for the switching and/or drive program override and the gearbox designed, as an electronic-hydraulic controller, is implemented electrically and with which an electronic control unit of the electronic-hydraulic controller is activated directly via the electrically active connection. The electrically activatable solenoid valves and electrically activatable pressure control valves are directly activated by the electronic control unit.
With an eventual failure of the electronic control unit, the solenoid valves, provided as actuators, and the electrical pressure control valves are automatically switched off. Thus, by way of pressurization by certain switching elements, assigned control valves are acted upon with hydraulic pressure so that the vehicle can be driven independently to a nearby workshop, albeit with restrictions on the operation of the automatic transmission.
For this purpose a multi-stage, dry-running operation is started, if an error is identified in an electrical gearbox, and the controller, active in the operation, is deactivated with the effect that the pressure control valves electrically activatable by the electrical gearbox mechanism are no longer activatable by the inactive (and thus switched off) gearbox mechanism. The multi-stage, dry-running operation of the electrically operated gearbox is provided, in order with high vehicle velocities, to put in the automatic transmissions a highest possible emergency or an emergency gear adjusted to the current vehicle velocity, which exerts only minor effects on a drive train of a vehicle.
Since with a high gear in the output of a vehicle only minor tractive forces are present, a start of a vehicle from standstill with high gear put in the automatic transmission is difficult or not at all feasible. For this reason, it is provided with an activated emergency operation, to put an emergency gear in the automatic transmission, which corresponds to a low gear of the automatic transmission in which, in the change from a high emergency gear to the low emergency gear, a restart of the vehicle is basically to be accomplished.
In order basically to prevent a false activation of such an automatic transmission, the pressure supply of the switching element first results after securing a trouble-free operation of the electronic gearbox, which causes a shift of a valve gate generally designated as a position valve mechanism via an electrical actuator.
In order to continue maintaining the pressure supply of certain switching elements during emergency operation in which the electrical supply of the gearbox is dispensed with and all electrical actuators are in their initial position, the electrical signal of an electrical actuator must be replaced by another signal available in the emergency program, since otherwise at least the last operating condition inserted or the gear range of the automatic transmission cannot be maintained.
Especially with controllers with short hydraulic circuits which, due to small volume, can offer no hydraulic elasticity, spring-operated accumulators are inserted by which, at least for a period of time, as long as the spring-operated accumulator is relaxed, the pressure in this circuit can be sustained with the failure of the electronics. As soon as such a spring-operated accumulator is emptied or the spring stops, the pressure supply to the clutch also collapses here. Such spring-operated accumulators are, therefore, only suitable to bridge short-term electronic failures, for example, for a time span of 20 milliseconds, during which a changeover of the valve would otherwise already occur.
The pressure of a clutch can be used as a hydraulic, spring-operated accumulator, whose pressure is also sustained with short-term under-supplies of the system pressure. For example, if a pressure accumulator function of 1 bar is selected with the construction of the position valve, the retaining function may not be above a clutch pressure of 1 bar.
This small retaining pressure results in the fact that in the operating gears “neutral” and “reverse drive”, the lock of the position valve mechanism is already activated with a very small, erroneously activated clutch-pressure and a pressure signal rests against the gearbox, which selects the operating gear “forward drive” in the automatic transmission, even though this does not correspond to the demand of the driver. That means that in the operating gears “reverse drive”, “neutral”, or “park”, locking the position valve mechanism is not desired and should be avoided as much as possible.
A device for the control of a hydraulically operable clutch of an automatic transmission by way of which the lock of the position valve mechanism selecting the operating gear “forward drive” in the automatic transmission is avoided with an inappropriate demand on the part of the driver, is known from DE 103 03 639 A1. For this reason, an electrically activatable actuator with a disturbance-related, cut-off is provided, so that the pressure in a first working chamber of the position valve mechanism is assigned to the transition into an emergency operation gear assigned to the current operation gear of the automatic transmission by way of a second piston. The second piston is connected via a second working chamber and a clutch area of a switching element or a switching element inserted in parallel to it and with a cut-off of the actuator forms a kind of damper. This damper holds the filling or the emptying of the switching element in the adjusting valve piston in a position assigned to the current operation gear for a given duration of time, within which a safe malfunction detection is executable via the electronic gearbox.
However, it is disadvantageous that the duration of time available for the suggested device is possibly inadequate for safe malfunction detection on the transmission side and switching elements are activated in the transmission, which do not correspond with an operating condition needing adjustment of the automatic transmission.
Therefore, the task which underlies the invention at hand is to make a device available for the control of a hydraulically operable switching element of an automatic transmission of a vehicle by way of which erroneous activations of an automatic transmission are safely avoided.